Educated Guess

I am quite sure there will be a lot of media attention on this item, which I caught over at Memeorandum. The New York Times is blaring a headline: Potential Flaw Is Found in Design of Fallen Bridge. Which is fine, the Federal Highway Administration has obviously raised a real red flag here. Here's the start of the article:

MINNEAPOLIS, Aug. 8 — Investigators have found what may be a design flaw in the bridge that collapsed here a week ago, in the steel parts that connect girders, raising safety concerns for other bridges around the country, federal officials said today.

The Federal Highway Administration swiftly responded by urging all states to take extra care with how much weight they place on bridges when sending construction crews to work on bridges. Crews were doing work on the deck of the Interstate 35W bridge when it gave way, hurling rush-hour traffic into the Mississippi River and killing at least five people.

The National Transportation Safety Board’s investigation is months from completion, and officials in Washington said they were still working to confirm the design flaw in the so-called gusset plates and what, if any, role it had in the collapse.

Still, in making public their suspicion about a flaw, the investigators were signaling they consider it a potentially crucial discovery and also a safety concern for other bridges around the country. Gusset plates are used in the construction of many bridges, not just those with a similar design to the one here.

“Given the questions being raised by the N.T.S.B., it is vital that states remain mindful of the extra weight construction projects place on bridges,” Secretary of Transportation Mary E. Peters said in a statement issued late today.

Nothing out of line here. But there is something missing and it matters a lot. What traffic load was the bridge originally designed for? There is a design criteria on file for that bridge, or someone did not do their job. The bridge was designed in the early to mid 1960's – how many cars per day was it expected to carry? What was it designed for? Reports say it was carrying 141,000 cars per day. I rather suspect that the design criteria will not project that load. There may – and quite likely is – a real problem here. But without that information it is very, very hard to judge. Taking an educated guess here, that bridge was probably not designed for the load that was on it daily (it really isn't the weight here, per se - that is limited by physical constraints – only so many cars can fit on there at one time. The vibrations of the traffic, however, do matter. Anticipated traffic speeds also matter. Weight is somewhat important since loads – especially on big trucks – have gone up. But there is a lot in play here from an engineering standpoint.) That bridge was also designed by engineers using slide rules (and healthy fudge factors). If you are reading this, you are working at a computer that engineers back then would have killed to get their hands on.